1. Field of the Invention
This invention relates to redundant power supply systems wherein one of the power supplies of the system is designed to be capable of providing the total required system power with the remaining power supplies being utilized for backup.
2. Brief Description of the Prior Art
The output terminals from redundant power supplies are normally connected together at the system load through diodes which decouple failed power supplies from the system by reverse biasing the diode in series with the failed power supply. The output voltage feedback signal for these power supplies is generally taken from the load side of the diode where precise voltage regulation is required.
A problem encountered in such power supply circuit arrangements is that minor variations in the output voltages among the individual ones of the redundantly connected power supplies are always present, this resulting in load power being supplied by the power supply having the largest (highest) output voltage. The power supplies in the system which have initially lower output voltages are in the standby mode and will display a continual reduction in their output voltages to near zero volts because the output voltage at the load always appears to the control loops of these individual power supplies to be too high, thereby causing the output voltages therefrom to continue to be reduced. Accordingly, since the output voltage at the load remains constant, the error voltage continues to drive the lower output voltage power supplies into an off condition. The feedback control loop is thus effectively opened because of blocked conduction due to the reverse biased series diode at the output of each of the standby power supplies. This effective shutdown of the power supplies operating in the standby mode causes many of the power supply self-tests to indicate failure conditions thereof when the power supplies are in fact capable of recovering to a fully functional status.
An additional problem may develop upon the sudden failure of the active power supply in the redundant configuration resulting in a transient voltage drop out as one of the power supplies operating in a standby mode recovers and assumes an active mode. The output filter capacitor for the standby power supplies would be nearly discharged and would require some amount of time to charge to the appropriate output voltage. This time would result in a voltage drop out condition at the load if the failure of the active power supply is sufficiently abrupt.